1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus for an internal combustion engine that includes a variable lift mechanism, which changes the lift amount of an intake value. The air-fuel ratio control apparatus also controls the air-fuel ratio based on a signal output from a sensor that detects the oxygen concentration in the exhaust gas present downstream of an exhaust gas purification catalyst.
2. Description of the Related Art
In an internal combustion engine, the exhaust gas is usually purified by an exhaust gas purification catalyst provided in an exhaust passage. The exhaust gas purification catalyst efficiently purifies the exhaust gas, when the air-fuel ratio of the air-fuel mixture, that is burned in the internal combustion engine, is within a predetermined range. Therefore, a sensor, which outputs a signal indicating the oxygen concentration in the exhaust gas, is provided upstream of the exhaust gas purification catalyst, and the actual air-fuel ratio of the air-fuel mixture is determined based on the signal output from the sensor. Then, the air-fuel ratio control is performed. In the air-fuel ratio control, the determined air-fuel ratio is compared with the target air-fuel ratio. If the determined air-fuel ratio deviates from the target air-fuel ratio, the fuel injection amount is corrected so that the actual air-fuel ratio becomes equal to the target air-fuel ratio.
Japanese Patent Application Publication No. JP-A-2004-36396 describes providing another sensor, which outputs a signal indicating the oxygen concentration in the exhaust gas, downstream of the exhaust gas purification catalyst to assess the condition of the exhaust gas purification action of the exhaust gas purification catalyst, in addition to the sensor provided upstream of the exhaust gas purification catalyst described above. Then, so-called sub-air-fuel ratio control, that is, an air-fuel ratio control that corrects the fuel injection amount based on the signal from the sensor provided downstream of the exhaust gas purification catalyst is performed.
Japanese Patent Application Publication No. JP-A-2001-263015 describes an apparatus that includes a variable lift mechanism, which adjusts the lift amount (more specifically, the maximum lift amount) of an intake valve of an internal combustion engine. With such apparatus, the operation of the variable lift mechanism is controlled so that the actual lift amount becomes equal to the target lift amount. Thus, the intake valve is opened or closed to achieve the lift amount appropriate for the current operating state of the internal combustion engine.
There is an individual difference in the lift amount of the intake value, due to an influence of, for example, a fitting error. Accordingly, the cross-sectional area of a communication portion (a part of an intake passage) in an internal combustion engine, at which an intake passage is communicated with a combustion chamber (hereinafter, such cross-sectional area will be referred to as the “passage area”), may slightly deviate from the reference passage area. Also, deposits may adhere to the intake valve with the operation of the internal combustion engine. In such a case, the passage area changes, causing the actual passage area to deviate from the reference passage area. Such deviation of the actual passage area from the reference passage area contributes to reduced accuracy in adjustment of the intake air amount, and, consequently, reduced accuracy in adjusting the air-fuel ratio of the air-fuel mixture. In the internal combustion engine where the above-described air-fuel ratio control is performed, the deviation of the actual air-fuel ratio from the reference air-fuel ratio due to the deviation of the actual passage area from the reference passage area is compensated for, basically, through the air-fuel ratio control. As a result, reduction in the accuracy in adjustment of the air-fuel ratio is avoided.
In the internal combustion engine provided with the variable lift mechanism, the deviation of the actual passage area from the reference passage area causes the actual air-fuel ratio to deviate from the reference air-fuel ratio. Such deviation of the actual air-fuel ratio from the reference air-fuel ratio varies depending on the manner in which the variable lift mechanism is operated. More specifically, operating the variable lift mechanism to reduce the lift amount of the intake valve increases the ratio of the deviation of the intake air amount from the reference value, due to the deviation of the actual passage area from the reference passage area, to the total intake air amount. Consequently, the deviation of the actual air-fuel ratio from the reference air-fuel ratio increases.
Accordingly, if the manner in which the lift amount changing mechanism is operated frequently changes so as to be appropriate for the current operating state of the internal combustion engine, the deviation of the air-fuel ratio from the reference air-fuel ratio also changes frequently. The apparatus described in Japanese Patent Application Publication No. JP-A-2004-36396 controls the air-fuel ratio based only on the signal from the sensor. With such an apparatus, if the manner in which the variable lift mechanism is operated changes so frequently, it may not be possible to accurately compensate for the deviation of the air-fuel ratio from the reference air-fuel ratio, which is frequently changed in accordance with a frequent change in the manner described above.